Spectroelectrochemical Investigations of Semiconductor Nanoparticles

The ability to tune the electronic band gap of semiconductor nanoparticles or “quantum dots” by controlling their size simply by variation of the synthetic conditions has opened many possibilities for applications across a wide range of fields. Many of these applications, such as solar cells, catalysis, sensing and light emitting diodes involve charge transfer processes between the nanoparticles and an adjacent phase. In order to make that charge transfer as efficient as possible, knowledge pertaining to the absolute energy positions of the electronic levels of such nanoparticulate materials is of primary relevance. The determination of these values and the important parameters that influence them was therefore the central issue of the present work. An electrochemical approach was chosen so that the data obtained could be referred to an absolute energy scale.The ability to tune the electronic band gap of semiconductor nanoparticles or “quantum dots” by controlling their size simply by variation of the synthetic conditions has opened many possibilities for applications across a wide range of fields. Many of these applications, such as solar cells, catalysis, sensing and light emitting diodes involve charge transfer processes between the nanoparticles and an adjacent phase. In order to make that charge transfer as efficient as possible, knowledge pertaining to the absolute energy positions of the electronic levels of such nanoparticulate materials is of primary relevance. The determination of these values and the important parameters that influence them was therefore the central issue of the present work. An electrochemical approach was chosen so that the data obtained could be referred to an absolute energy scale. To achieve reliable measurements a new strategy was developed so that dense and homogeneous monolayers of semiconductor particles could be deposited onto transparent electrodes. The films were obtained by exchanging the original bulky ligand shell of the nanocrystals with a reactive alkoxysilane species and subsequent immersion of the substrate into a solution of the modified nanocrystals. SEM and electrochemical investigations have shown a much higher coverage efficiency in comparison with other methods presently established in the literature, which are based on the approach of prefunctionalizing of the substrates prior to coating. Fractional coverages of 80 % were obtained within 24 h while avoiding the time consuming and complicated step of functionalizing the substrates before deposition. Films of CdSe and CdS nanoparticles deposited on fluorine doped tin oxide (FTO) electrodes were characterized by means of potential modulated absorption spectro-scopy (EMAS). Employing this special spectroelectrochemical technique, bleach signatures in the absorption spectra of the quantum dots induced by electron injection into their respective conduction band states were investigated. The features observed in the spectra and the evaluation of the potential dependence of the signal intensity revealed that only the lowest conduction band state, namely the 1Se state, is populated. The occupancy follows a quasi Fermi-Dirac distribution whose distributional width, in addition to the temperature, also depends on the size distribution of the particle ensemble investigated. On that basis a model was developed to extract the electrochemical potentials of the respective populated lowest conduction band states. For CdSe quantum dots the four energetically lowest excitonic transitions were found to become bleached as the 1Se state is populated, indicating that these transitions promote electrons from different states in the valence band to the same conduction band state. These findings are in excellent agreement with results obtained from ultra fast optical pump probe experiments, which are methods that usually demand much more experimental efforts than the technique presented in these studies. The determination of the potential of the 1Se state versus a known reference potential allows one to map the top valence band states with respect to an absolute energy scale. This provides the opportunity to compare the energy positions obtained for different samples. Determination of the electrochemical band edge potential clearly features a size dependent shift of the conduction band edge and the valence band edge for both CdSe and CdS quantum dots, which is in excellent agreement with the expected behavior due to the quantum confinement effect. Investigations in different electrolytes have shown that the immediate environment has a major impact on the electrochemical potentials of the energy levels of the nanoparticles. This observation is particularly important from a technological point of view, as in many applications the semiconductor material is in direct contact with an electrolyte as for example in quantum dot sensitized solar cells, electrochemical sensors and catalysis. In contrast to other “purely physical” methods such as photoelectron spectroscopy or scanning tunneling spectroscopy, potential-modulated absorption spectroscopy provides the ability to probe the materials under their most likely “working” conditions where such environmental influences can be directly taken into account. Further, it has been shown that potential modulated absorption spectroscopy can be applied to bulk semiconductor electrodes, as long as they are thin enough to allow adequate amounts of light to pass through. The features observed in the EMAS spectra of these samples clearly differ from those obtained for nanoparticle films, as in such materials a continuum of states is progressively filled rather than a single state. Besides band-filling the potential modulation additionally induces changes in the absorption, which can be attributed to the Franz-Keldysh effect resulting from the modulation of the electric field across the space charge layer. The resolution and sensitivity that one can obtain with this comparatively simple and cost-effective setup is quite remarkable. As has been demonstrated it was possible to achieve clearly resolved bleach spectra of submonolayers of quantum dots attached to FTO with optical densities below 0.001. Recently it has been reported that cyclic voltammetry (CV) can be used to study the size dependent positions of the electronic levels of quantum dots. The intention of the last part of this thesis was to reproduce this work for the nanoparticles investigated within this thesis in order to compare the results with those obtained by EMAS. However, the experiments undertaken here reveal that the anodic and cathodic peaks observed in the cyclic voltammograms cannot automatically be assigned to the absolute band edge positions of the particles as the size dependent peak positions and their potential differences do not show any evidence for a correlation with respect to the quantum size effect. Rather the voltammetric responses reflect the solid state electrochemical characteristics of CdSe. Theoretical considerations concerning the response expected in a CV due to band filling of semiconductor nanoparticles confined to an electrode surface revealed that the expected currents are quite similar to that of a pseudo-capacitance. However, pronounced signals are only obtained if appropriate amounts of deposited nanoparticles are present which are electronically addressable without hampering the charge transfer. Hence a clear assignment of the peaks obtained in a cyclic voltammogram to the electronic band edges without employing a complementary technique to confirm ones findings therefore seems to be at best questionable